If your not doing the storyline mode (or campaign, whatever) There are two easy maps that allow you to survive for a long time. 1) The acropolis map - You can just wall off the two entrances to your little hill and defend them easily, since your enemy has only two ways to get in. 2) The river nile - Make a shit-ton of enemies and wall off your little land mass from the river, then line that wall with towers. The closer to the river, the better, troops won't be able to land and you can destroy almost any ship that comes nearby. Both these maps I played on for hours and hours on any difficulty level. They give you plenty of time to upgrade and prepare to assualt each enemy at your convience.